Dark current refers to an undesired signal generated by a pixel of an imaging device even in the absence of a light signal. One source of dark current is thermally-generated electrons and holes. Thermally generated dark current in a CMOS active pixel imaging device presents problems in many imaging applications. As temperature increases, dark current likewise increases. Thermal energy generates unwanted free electrons in pixels. These unwanted free electrons alter the desired signal produced by electrons generated by incident photons. Additionally, some digital still cameras require long integration (exposure) times, which allows for higher image sensitivity. However, the longer the integration time the higher the sensitivity to thermally generated dark current.
The largest portion of unwanted free electrons in a pixel array occurs at the interface between a silicon dioxide layer overlying a substrate and an epitaxial or other silicon substrate layer of a chip on which pixel devices are formed. Even in an ideal defect-free process, dark currents also exist due to generation/recombination centers in the depletion region of a photodiode employed as the light sensitive element of a pixel. There are thermodynamic lower limits for dark currents, which place limitations on light sensitivity. As pixel technology is scaled down for high-density mega-pixel arrays for single chip digital cameras, sub-micron CMOS technology process-induced defects increase dramatically. Stress, extended defects and metallic impurities among other factors all contribute to undesirable dark current.
For example, the transfer, reset, source follower transistors typically employed in a four transistor (4T) pixel must be scaled down to achieve higher pixel density. Such scaling is not possible without a corresponding increase in dopant levels (e.g., p-well, Vt-adjustment and halo implant) and a reduction in gate oxide thickness for the transistors. This leads to higher levels of junction leakage, gate induced drain leakage (GIDL), drain-induced barrier lowering (DIBL) among others. All of the above contribute to undesired dark current levels. Additionally, field isolation region pitch must also be scaled down leading to increased stress levels and electron leakage near the field-edge of the photodiode.
Dark current for a given pixel actually has two components, a static portion and a variable portion. The variable portion is due to shot noise and follows Poisson statistics, with the root mean square (rms) dark current noise being equal to the square root of the dark current.
One method of compensating for dark current has been to identify an average pixel dark current and subtract that average value from each pixel signal. This method, however, fails to provide accurate and realistic images and ignores dark current noise, which is generated from outside of the image area. This technique is not repeatable and does not work well for most CMOS image sensors.
Defective pixels are another problem with imagers. Defective pixels include hot pixels, bad pixels and dead pixels. Hot pixels have a higher than average dark current, which arises from the manufacturing process so the dark current caused by each hot pixel remains fixed and can be corrected. Hot pixel information is recorded with dark current information and dark current compensation will also compensate for hot pixels. Bad pixels are pixels that are bad when manufactured or have an indication that they may go bad during their life. Detection of bad pixels is accomplished by capturing images of a uniformly lit background with different integration times. In particular, as the integration time is increased, a well-behaved (i.e., good) pixel is expected to produce a signal which increases linearly as a function of the increasing integration time. Pixels that behave in a non-linear fashion (for example, respond to linearly increasing integration times with spurious values) are identified as bad pixels. Dead pixels are pixels that do not respond correctly to light input. These pixels may appear as a colored bright dot or a dark dot on an image depending on the failed mechanism. There are three forms of dead pixels; dark dead pixels, white dead pixels and saturation dead pixels. Dead pixels can be detected either by hardware or by software after a pixel array is fabricated. A dark dead pixel is detected when the dark threshold value is above an average value of normal pixels under no light condition. A white dead pixel is detected when the white threshold value is above or below an average value for normal pixels under, for example, approximately a 28 Lux light condition. A saturation dead pixel is detected when the saturation threshold is below an average value of normal pixels under, for example, approximately a 100 Lux light condition.
Defective pixels, such as hot pixels, dead pixels and bad pixels are created during the manufacturing process may occur during the life of an imaging device containing a pixel array. Yield considerations and replacement costs for pixel arrays dictate that an alternative to discarding pixel arrays or replacing cameras or pixels arrays of imaging devices be provided.